Engine timing systems typically include an endless chain wrapped around a driving sprocket on an engine crankshaft and a driven sprocket on an engine camshaft. The rotation of the crankshaft causes the rotation of the camshaft through the endless chain system. A second sprocket mounted upon the crankshaft may be used to drive a balance shaft system using a separate endless chain.
More complicated engine timing systems connect the crankshaft with two or more shafts by a pair of chains. The crankshaft includes two sprockets. Each chain is connected to one or more driven sprockets, including sprockets on each of the two overhead camshafts. Typically, the chain systems in more complicated engine timing systems will include tensioners on the slack side of each chain to maintain chain tension and snubbers on the tight side of each chain to control chain movement during operation.
Some engine timing systems have two (or dual) overhead camshafts for each bank of cylinders. The dual camshafts on a single bank can both be rotated by connection to the same chain. Alternatively, the second camshaft can be rotated by an additional camshaft-to-camshaft chain drive. The cam-to-cam drive chain can also include single or dual tensioners for chain control.
In some engine timing systems, such as those having a non-conventional firing order for the cylinders, balance shafts are employed to counterbalance engine vibration. The balance shafts are driven by a chain connection from the crankshaft. Optionally, the balance shaft drive system may be utilized to operate an auxiliary drive such as a compressor or the like. Since the balance shafts are driven by the crankshaft, torsional vibrations and oscillations along the crankshaft may be transferred to the balance shafts and likewise throughout the timing system.
The rotating crankshaft may undergo resonance at certain frequencies. Since the balance shafts are coupled to the crankshaft by the balance shaft chain, the balance shafts are directly exposed to these extreme resonant torsional vibrations. Vibrations from the resonance of the crankshaft are often transferred throughout the system, including the balance shafts and associated engine elements and can significantly increase the load on the systems and components, increase the noise from the engine and increase wear and accelerate fatigue of the chains.
Conventional approaches to this problem have focused on reducing rotational perturbation of the crankshaft, by means of internal devices such as counter-rotating balance shafts, Lanchaster dampers and harmonic balancers. External devices such as fluid engine mounts and engine mounts having adjustable damping characteristics have been used.
By contrast, the present invention focuses on absorbing the torsional vibrations of a crankshaft using a torsionally compliant and damped sprocket system. The torsionally compliant and damped sprocket system minimizes the transfer of such vibrations and torque spikes to other parts of the engine system. The torsionally compliant sprocket system interposes resilient members between the drive sprockets mounted on the crankshaft to absorb vibrations and reduce transfer of the crankshaft vibrations. The present invention, in addition, provides a damping mechanism to add sufficient damping to reduce or eliminate vibrations associated with resonant frequencies in the system.
Some prior art timing systems use various damping devices to address the problem of vibrations. One example of such prior art system uses a rubber damper piece which is placed against a sprocket and bolted to the shaft to absorb vibrations. However, the rubber damper piece may fracture from the extreme resonance vibrations. Other timing systems employ a weight that is positioned on the shaft and held against the sprocket by a Belleville washer. Frictional material is also placed at the area of contact between the sprocket and the weight. These systems, while effective at damping have drawbacks in terms of production, assembly and durability. None apply damping to a compliant sprocket.
An example of the above-described prior damping techniques is found in Wojcikowski, U.S. Pat. No. 4,317,388, which issued on Mar. 2, 1982. That patent discloses a gear with split damping rings of diameter slightly smaller than the gear bolted to each side of the gear with a tapered bolt and nut assembly. Tightening of the bolt cams the damping ring outward, producing pressure circumferentially against the rim of the gear and causing tensile stress on the gear. Additionally, tightening of the bolts presses the elastomeric washers associated with the bolt and nut assembly firmly against the web of the gear which damps the stress wave passing from the rim through the web and into the shaft. In contrast to this prior art structure, the present invention utilizes a novel arrangement of sprockets to produce a torsionally compliant sprocket assembly to reduce the transfer of vibrations of the crankshaft to other parts of the engine timing and balance shaft drive system.
Another example of the above-described prior art is Funashashi, U.S. Pat. No. 5,308,289, which issued on May 3, 1994. The damper pulley disclosed therein consists of a pulley joined to a damper mass member with a resilient rubber member. The pulley and the damper-mass member each have at least two projections, and the projections of the pulley contact the sides of the projections of the damper mass member. A second resilient rubber member is placed between the contacting projections. Bending vibrations from the crankshaft cause the pulley to vibrate in the radial direction and the first resilient rubber member deforms, causing the dynamic damper to resonate with the pulley and restrain the bending vibrations. Torsional vibrations cause the pulley to vibrate in the circumferential direction. The second resilient rubber member undergoes compression deformation, decreasing the spring force and raising the resonance frequency against the torsional vibrations. Compliance for a sprocket used in a timing drive system or balance shaft drive system is not contemplated. The present invention avoids the use of rubber which has wear problems in use.
Another example of a prior damping technique is found in Kirschner, U.S. Pat. No. 4,254,985, which issued on Mar. 10, 1981. That patent discloses a damping ring for rotating wheels that includes a viscoelastic damping material disposed within an annular groove in the surface of the wheel. A metal ring is positioned in the groove at the top of the damping material. In operation, the damping material undergoes shear deformation. The technique is applied to train car wheels to reduce brake noise.
Still yet another example of prior art damping techniques is found in U.S. Pat. No. 4,139,995 which discloses a high deflection amplitude torsional vibration damper for use in a torsional coupling between a driving member and a driven member. The damper includes a hub receiving a driven shaft and having oppositely disposed arms. The damper has a pair of equalizers with oppositely extending arms journal led on the hub. A pair of cover plates encloses the assembly and has integral driving means formed therein. A plurality of compression springs are found within the plates, positioned between the hub and equalizer arms. The technique is disclosed being used in a drive train clutch but not a compliant sprocket.